The invention relates to a circuit for tunable RF-circuits, for example for tuning an oscillator, comprising to compensate for temperature-responsive variations in the frequency owing to the temperature coefficients of the components, a temperature-responsive voltage divider having a tap which is connected to a tuning voltage source via a tuning diode.
Such a circuit arrangement is disclosed in German published Patent Application No. 1,614,794.
In the known circuit a tuning voltage, which is supplied by the tuning voltage source via a controllable tuning voltage divider, is applied to the cathode of the tuning diode. The voltage across the tuning diode is compensated for temperature variations by means of a compensation diode, which is connected between the tuning diode anode and ground. In conjunction with a series-arranged series resistor, the compensation diode forms a temperature-responsive voltage divider which is supplied, in the prior art circuit, from the tuning voltage source.
However, partly because of the severe requirements imposed on the stability and on the rangewidth of the tuning voltage, the output power of the tuning voltage source is very limited. This particularly applies when the tuning voltage source has been realized in the form of an integrated circuit. The energy which the temperature-responsive voltage divider takes from the tuning voltage source must therefore be kept low, which in practice results in that the current through the compensation diode is too limited to effect a temperature-responsive voltage variation with the compensation diode which compensates to a sufficient degree for the temperature-responsive voltage variation across the tuning diode.